AC Fundamental Interview Question Answer - 3

Important Question of AC Fundamentals

In this post, some of the important question of resonance, definition of impedance, admittance, inductive susceptance, inductive reactance, parallel resonance, impedance of series RL circuit, series resonance frequency, parallel resonance frequency, rejector circuit, current resonance, q factor of parallel resonance circuit and q factor of series resonance circuit.

Resonance Power

Describe the relation between resonance power and off resonance power.         

The off-resonance power = P / ( 1 + Q² )

Where P = Power at resonance and

           Q = Tangent of the circuit phase angle

The off-resonance power is reduced by factor ( 1 + Q² ) to that of resonance power.

AC Fundamental Definitions

Explain the following terms: Impedance, Admittance, Conductance, Inductive reactive, Inductive susceptance, Capacitive reactance and Capacitive susceptance

Impedance ( Z )

• It is defined as the combined effect of resistance and (inductive or capacitive) reactance. Its unit is ohm.

           Z = r + j X_L    

                 OR

           Z = r – j X_C

Admittance ( Y )

• The reciprocal of impedance is called as admittance. Its unit is Siemens.

        Y = 1 / Z

Conductance ( G )

• The reciprocal of resistance is called as conductance. Its unit is ohm^-1.  

        G = 1 / R

Inductive reactive ( X_L )

• The opposition offered by the inductance to the flow of current is called as inductive reactance.
• It is donated by X_L = 2πfL

Inductive susceptance ( B_L )

• The reciprocal of inductive reactance is called as inductive susceptance. The inductive susceptance is taken as negative because the inductive reactance is taken as positive.

       B_L=1 / X_L

Capacitive reactance ( X_C )

• The opposition offered by the inductance to the flow of current is called as inductive reactance.
• It is donated by X_C = 1 / 2πfC

Capacitive susceptance ( B_C )

• The reciprocal of capacitive reactance is known as capacitive susceptance. The capacitive susceptance is taken as positive because the capacitive reactance is taken as negative.

       B_C =1 / X_C

At which condition the conductance is reciprocal of resistance and susceptance is reciprocal of inductive reactance in the series RL circuit?

Impedance of series RL circuit

= R + jX_L

= ( R + jX_L ) / ( R² + X_L² )

= [ R / ( R² + X_L² )] + j [ X_L / ( R² + X_L² )]

= G + jB

 Therefore

 G = [ R / ( R² + X_L² )]

 B = [ X_L / ( R² + X_L² )]

• When the inductive reactance becomes zero, the conductance is reciprocal of resistance. 
• Similarly, when the resistance becomes zero, the susceptance is reciprocal of inductive reactance.

Parallel Resonance

Describe the condition for parallel resonance.        

Parallel resonance

• The circuit is called as parallel resonance when the net reactance of the circuit becomes zero.

Give reason: The parallel resonance is referred as current resonance.

Current resonance

• The parallel resonance is called as current resonance because current circulates between two parallel branches is much higher than the supply current.

Define : Parallel resonance frequency  

Parallel resonance frequency

• It is frequency at which net reactive component of the circuit becomes zero.

        f_r = √ [( 1 / LC ) – ( R² / L² )]

Give reason: The parallel resonance circuit is called as rejector circuit.

Rejector circuit

• The parallel resonance circuit is called as rejector circuit because it rejects that frequency with it resonates.

Describe the effect of frequency on the susceptance of the parallel resonance circuit.        

Capacitive susceptance

• It is directly proportional to the applied frequency. 
• As the frequency increases, the capacitive susceptance increases.

Inductive susceptance

• It is inversely proportional to the applied frequency. 
• As the frequency increases, the inductive susceptance decreases.

Q Factor of Parallel Resonance

Explain the term: Q factor of parallel circuit 

Q factor of parallel circuit

• It is defined as the current circulating between two parallel branches to the supply current.   

Describe the significance of Q factor in the series resonance circuit and parallel resonance circuit

Significance of Q Factor

• The Q factor in the series resonance indicates voltage magnification whereas it indicates current magnification in the parallel resonance circuit.

Series Resonance Frequency

Define: Series resonance frequency      

Series resonance frequency

• It is frequency at which net reactance of the circuit becomes zero.

         f_r = 1 / 2π √ ( LC )

 

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